what is Microsoft security essentials do I need to Buy it ?

Microsoft Security Essentials (MSE) is an antivirus software (AV) product that provides protection against different types of malware such as computer viruses, spyware, rootkits and Trojan horses. It runs on Windows XP, Windows Vista and Windows 7, but not on Windows 8, which has a built-in AV component. THE LICENSE AGREEMENT ALLOWS HOME USERS AND SMALL BUSINESSES TO INSTALL AND USE THE PRODUCT FREE OF CHARGE. It replaces Windows Live OneCare, a discontinued commercial subscription-based AV service, and the free Windows Defender, which until Windows 8 only protected users from ad-ware and spyware.